The present invention relates generally to diagnostic methods and circuits for commutated relay switching circuitry. More particularly the invention relates to a method and circuits for automatically detecting the failure, either open or closed, of a master relay, commutating diodes, and the pilot relays in a commutated relay switching arrangement.
Conveniently and by way of example the invention may be employed in household cooking appliances incorporating multiple heating units, such as ranges and cooktops. Commonly assigned U.S. Pat. No. 296,449 to Charles W. Eichelberger, discloses a commutated relay switching arrangement which is advantageously used in such cooking app1iances. In that arrangeaent each of four surface units is serially connected to the relay contacts of an associated pilot relay. The serially connected pilot relay contacts-surface unit combinations are connected to each other in a parallel array. A single master relay having contacts connected in series with the parallel array couples the array to the power supply. The master relay is shunted by a commutating diode which serves to commutate the current flow through the shunted relay contacts. By opening and closing the master relay during the half-cycle of the AC power signal during which the diode is forward biased the master relay is not subject to substantial power levels during the transition period. The pilot relay contacts are switched during the next occurring half-cycle of AC power signal during which the commutating diode is reverse biased. During this half-cycle substantially no current flows in the pilot relay contacts circuit since the master relay has already been opened during the previous half-cycle and the diode is reverse biased. Consequently, the voltage requirements for the pilot relay are very low and a relatively inexpensive relay may be used as the pilot relay. However, as noted in the Eichelberger patent if the commutating diode operates improperly the life of the master relay contacts will be significantly shortened. Similarly, if the master relay contacts fail closed, the possibility that the pilot relay contacts will fail increases. If both master contacts and pilot contacts fail in a shorted condition the power will be applied to the associated load when not desired.
To address these problems Eichelberger provides fail safe circuitry to perform two tests on the commutating circuitry. The first test determines if the master relay is working, and the second determines if the commutating diode is intact. Each test is performed during the latter half of the positive half-cycle of the power signal after the appropriate master relay contacts have been opened but before the appropriate pilot relay contacts have been changed. If at the end of the test half cycle it is determined that the master relay contacts have failed closed or the commutating diode has failed open, the switching cycle is aborted and no change is made to the state of any of the pilot relay contacts. The microprocessor may also generate an alarm which indicates the system is working improperly and should be shut off and/or repaired.
An additional protective feature is provided in the form of a slow blow fuse connected in series with the commutating diode. If for any reason the master relay contacts are not closed for a long period of time while any one or more of the pilot relays remains closed, the slow blow fuse will open and power will be removed from the load.
While the diagnostic and protective features disclosed by Eichelberger provide significant protection, there are inherent limitations. For example, the state of the pilot relays when the tests are performed is unpredictable and certain states, such as the state where all pilot relays are open at the time the master relay is to be switched, may provide misleading test results, resulting in unnecessarily aborting the switching cycle. Also, the testing does not diagnose failure of the master relay in the open mode, that is failure to close properly. Instead a fuse is employed which adds extra cost and complexity to the power circuit. Furthermore, no provision is made for testing the pilot relays for failure in either the open or the closed mode. Finally, failure of the master relay or diode adversely affects all four surface units.
It would be highly desirable therefore to provide power control switching and diagnostic circuitry which reliably detects master relay, commutating diode, and pilot relay failures in either a short mode or an open mode, and which is further operative upon detection of failure of any of these components in the short mode, or master relay or diode failures in the open mode to de-energize only that portion of the control circuit affected by the failed component and to provide a user discernible signal indicating that such failure has been detected while permitting normal operation of that portion not affected by the failure. It would be further desirable to provide a switching circuit in which a single failure of a master relay or commutating diode does not affect all of the surface units.
It is therefore an object of the present invention to provide power control and diagnostic circuitry which automatically periodically tests for either master relay or commutating diode failures in the short mode and which tests for master relay and diode failures in the open mode and finally which tests the pilot relays for failure in either the open or closed mode.
It is another object to provide circuitry of the aforementioned type which is operative upon detection of a failure of any of the relay or diode components in the short mode to de-energize those surface units affected by the failure and provide a user discernible signal informing the user that such a failure has been detected.
It is yet another object of the present invention to provide circuitry of the aforementioned type which provides a user discernible signal in the event one or more of the pilot relays fails in the open mode which signal informs the user which one of the surface units has failed in that mode, without affecting normal operation of the remaining surface units.